The latest arrival also made Samajwadi Party (SP) chief Mulayam Singh Yadav a great grandfather, sources told IANS.
Tej Pratap is the grand nephew of the Yadav patriarch.
The Mainpuri Lok Sabha member married Raj Laxmi, daughter of Rashtriya Janata Dal (RJD) supremo Lalu Prasad Yadav, in 2015.
The baby boy was born in a Delhi hospital late Wednesday.
What added to the joy apparently is the arrival of a baby boy to Misa Bharti, who is the eldest daughter of Lalu, on the same day.
Misa Bharti is a Rajya Sabha member from Bihar and has shared the news on social media.